Contrary to its name, stainless steel is not literally “stainless steel”. It is so called because it can withstand more time and use before showing signs of wear compared to ordinary steel. However, not all stainless steel is ‘stainless’ to the same degree.
While they are all alloys of iron and carbon (which is what ordinary steel is) with a significant amount of chromium added to them, which is what makes them resistant to corrosion, depending on their atomic structure and the composition of the alloy (including the addition of more material), they will be more resistant to corrosion or will have other properties that make them more suitable for certain applications or others.
Austenitic stainless steels, for example, are particularly resistant to corrosion, unlike ferritic, martensitic and duplex stainless steels. They are the most common and are achieved by adding sufficient austenitic stabilising elements of nickel, manganese and nitrogen, which makes them less susceptible to hardening during heat treatment and non-magnetic. The two most common austenitic stainless steel references are the so-called 304 stainless steel and 316 stainless steel.
300 series austenitic stainless steels
Austenitic stainless steels are divided into two main groups, namely the 200 series and the 300 series. The 200 series achieve their austenitic structure mainly on the basis of manganese and nitrogen, although they contain a small amount of nickel, whereas the 300 series is essentially achieved by the addition of nickel.
The 300 series of stainless steels is the largest sub-group of austenitic steels and within this series the most common is Type 304, also known as 18/8 or A2 steel.
Both 304 stainless steels and another widely used reference of the same series, 316 stainless steels, contain high levels of chromium and nickel (we will see in the next section what proportion of each is related to their weight, depending on the type of steel), which makes them particularly resistant to corrosion. They also contain important alloying elements: carbon, manganese, phosphorus, sulphur and silicon, both in the same proportion by weight as the two steels. The rest of the chemical composition is essentially iron.
304 stainless steel vs 316 stainless steel: the difference between the two qualities
To the casual observer, the difference between one grade of stainless steel and another is imperceptible, as we have commented before. However, for a fabricator the difference between stainless steel alloys, such as grade 304 stainless steel and grade 316 stainless steel, can be huge.
The key difference between these two grades of steel is that 316 stainless steel contains molybdenum, in addition to all the alloying components of iron, carbon, chromium and nickel and all the elements described in the previous section. Molybdenum substantially increases corrosion resistance, especially in environments with the highest salt content or chlorine exposure.
This is why 316 stainless steel is often referred to as “acid-proof steel”, a quality that makes it more suitable for applications in structures near the sea or even underwater, where, despite 304 stainless steel, it has considerable corrosion resistance due to its slightly higher proportion of chromium. In conclusion, although both 304 and 316 stainless steels are suitable for extreme environments, only the latter can withstand prolonged exposure to chlorides.
| Inox 304 | Inox 316 | |
| Carbon | 0,08% máx. | 0,08% máx. |
| Manganese | 2,00% máx. | 2,00% máx. |
| Phosphorus | 0,045% máx. | 0,045% máx. |
| Sulphur | 0,030% máx. | 0,030% máx. |
| Silicon | 1,00% máx. | 1,00% máx. |
| Chromium | 18,00%-20,00% | 16,00%-18,00% |
| Nickel | 8,00%-10,50% | 10,00%-14,00% |
| Molybdenum | – | 2,00%-3,00% |
Applications of stainless steel 304
As mentioned above, stainless steel 304 is the most common austenitic steel. Its nickel content is usually between 8 and 10.5% by weight and its chromium content is usually between 18 and 20% by weight. These high percentages give stainless steel 304 a high resistance to corrosion. In addition, it contains important alloying elements such as manganese, silicon and carbon, just like stainless steel 316. The rest of the chemical composition is essentially iron.
For outdoor furniture such as railings and bollards, stainless steel is an ideal material for corrosion resistance, but will only withstand prolonged exposure if the quality is suitable for its environment. 304 is an economical and practical choice for most environments, but does not have the chlorine resistance of 316.
Other examples of its application are.
Sinks and backsplashes.
Pots and pans.
Tableware and crockery.
Architectural panels.
Sanitary ware and sinks.
Plumbing.
Brewing, dairy, food and pharmaceutical production equipment.
Springs, nuts and bolts.
Applications of Stainless Steel 316
This is a stainless steel very similar to grade 304, also due to the high levels of chromium and nickel in its chemical composition. However, stainless steel 316 contains a significant amount of molybdenum, ranging from 2% to 3%, which makes it more corrosion resistant than 304 stainless steel and therefore suitable for extreme applications, such as marine applications, and affects the price of the material.
This slightly higher price of 316 stainless steel compared to 304 stainless steel pays off in areas of high chlorine exposure, particularly in coastal areas and in infrastructure with high salinity, such as coastal roads. In such applications, 316 stainless steel will last longer than 304 stainless steel, which can mean many additional service lives. However, its molybdenum content has a detrimental effect on formability.
Common applications for this type of steel include.
Food preparation equipment, particularly in chlorinated environments.
Worktops and laboratory equipment.
Coastal building panels, grilles and mouldings.
Boat fittings.
Chemical vessels, including for transport.
Heat exchangers.
Woven or welded screens for mining, quarrying and water filtration.
Threaded fasteners.
Springs.
How do you know if a steel is stainless steel 304 or stainless steel 316?
You cannot distinguish between stainless steel 304 and stainless steel 316 with the naked eye, as there is no obvious difference between two identical pieces of metal, one polished or textured in exactly the same way by one material or the other. To prove that a stainless steel is inox 304 or inox 316, a material test report (MTR) for the actual material is required. Some small parts made of 316 stainless steel, such as clips and mounting feet, are specially marked.
304 stainless steel or 316 stainless steel: which one to choose?
Each stainless steel application has its own unique requirements and requires a stainless steel that is up to the task.
In some cases, 304 stainless steel may be the best choice.
Those where excellent formability is required.
Those where high formability is required.
Those on a more limited budget and need a material that is affordable.
Some of the situations where 316 stainless steel may be the best choice are.
Environments that include a large number of corrosive elements.
Those where the material will be placed in water or exposed to water on a regular basis.
Those where greater resistance and hardness is required.